1) You are correct in that you need to move the volume which houses the LUN in order to move it to a new disk location. If you are running Data ONTAP 7-mode you can move a LUN-based volume hot to another aggregate (aka disk location) as long as it belongs to the same physical controller. If you need to move the LUN-based volume to another controller you could utilize SnapMirror however that would require downtime. I would strongly recommend looking into Clustered ONTAP as you can now move LUN-based volumes between physical filers without any disruption (hot).
2) When it comes to LUN-based volumes I typically only have one LUN per volume for the reason you mentioned. I have ran into a few scenarios where having this volume structure has helped simplify data migration activities.